The present invention is related to a method and a circuit for switching a source of a modulation frequency, and more particularly for switching a source of a modulation frequency used in a direct current to direct current (DC to DC) converter.
The traditional DC to DC converter is applied to adjust the voltage level of the DC input voltage to stabilize the output voltage at a pre-determined value.
Please refer to FIG. 1, which is a block diagram showing the circuit configuration of the prior art of the present invention. This circuit configuration is used to offer a constant frequency from a resistance-capacitance oscillator (RCO) 10 to a pulse width modulation (PWM) circuit 11 to engage in the modulation. Thus, a switching frequency signal Fp is generated to control a controllable switch 12 which is made of an n-type metal-oxide-semiconductor transistor (NMOS) to maintain the output voltage at a certain value.
The method to control the switching frequency signal Fp is as follows:
1. When Fp is at the high level, the controllable switch 12 is conducted. Through the inductor L, the input current IL flows through NMOS, then IL is grounded. Thus, the diode D is reverse-biased, and an output current is offered by the capacitor C onto the load RL.
2. When Fp is at the low level, the controllable switch 12 is closed. The inductor L will change the direction of the magnetic field thus making the positive pole and negative pole of the inductor to switch. This will cause the diode D to be forward-biased, and make the energy store in the inductor to produce an output current. Through the diode D, the output current is offered onto the load RL.
When the output voltage of the direct current to direct current (DC to DC) converter is stabilized at a pre-determined voltage, it will drive a system to begin the normal operation. Since the resistance-capacitance oscillator 10 will keep on offering a certain frequency to the pulse width modulation circuit 11 after the system is operated normally, therefore, the operation of the resistance-capacitance oscillator 10 will increase the power consumption of the whole system.
Keeping the drawbacks of the prior art in mind, and employing the experiments and research full-heartily and persistently, the applicants finally conceived the method and circuit for switching source of modulation.
It is therefore an object of the present invention to propose a method for switching a source of a modulation frequency used in a direct current to direct current (DC to DC) converter.
It is therefore another object of the present invention to propose a circuit for switching a source of a modulation frequency used in a direct current to direct current (DC to DC) converter.
According to the aspect of the present invention, the DC to DC converter has an oscillator and generates an output voltage to drive a system. The modulation frequency is used for modulating a pulse width modulation (PWM) circuit in the DC to DC converter. The method includes steps of:
1. providing the modulation frequency from the oscillator;
2. switching the source of the modulation frequency from the oscillator to a system clock to provide the modulation frequency when the system is driven by the output voltage from the DC to DC converter; and
3. stopping the oscillator for reducing a power loss.
Certainly, the oscillator can be a resistance-capacitance oscillator (RCO).
Certainly, the system can be a wireless communication system.
Preferably, the system clock is generated by a clock generator.
Preferably, the clock generator is mounted in the interior of the system.
Preferably, the step of switching the source of the modulation frequency is performed by a multiplexer.
Preferably, the multiplexer switches the source of the modulation frequency from the oscillator to the system clock in response to a system stabilized signal from the system, and then stops the oscillator.
Preferably, the method further includes the step of increasing an oscillating frequency of the oscillator, so as to increase the level of the output voltage rapidly when the output voltage is under a reference voltage to drive the system.
Preferably, the method further includes the step of reducing the oscillating frequency of the oscillator in order to reduce the power loss generated by the oscillator when the output voltage is over the reference voltage.
Preferably, the reference voltage is predetermined by users.
Preferably, the modulation frequency is a constant frequency.
According to the aspect of the present invention, the DC to DC converter generates an output voltage to drive a system. The modulation frequency is used for modulating a pulse width modulation (PWM) circuit in the DC to DC converter. The circuit for switching a source of a modulation frequency includes an oscillator for providing the modulation frequency, a clock generator providing the modulation frequency, and a first multiplexer electrically connected between the oscillator and the clock generator. The first multiplexer allows the oscillator to provide the modulation frequency before the output voltage is used to drive the system. The source of the modulation frequency is switched from the oscillator to the clock generator by the first multiplexer according to a system stabilized signal generated from the system after the system is driven by the output voltage.
Certainly, the oscillator can be a resistance-capacitance oscillator (RCO).
Certainly, the system can be a wireless communication system.
Preferably, the clock generator generates a system clock to provide the modulation frequency.
Preferably, the system stabilized signal is used to stop the oscillator.
Preferably, the oscillator further includes a second multiplexer mounted therein.
Preferably, the second multiplexer is used for selecting an oscillating frequency of the oscillator in response to an output signal of a comparative circuit.
Preferably, the comparative circuit is used for providing the comparison between the output voltage and a reference voltage in order to generate the output signal.
Preferably, the reference voltage is predetermined by users.
Preferably, the modulation frequency is a constant frequency.